Automatic developer controller

ABSTRACT

Apparatus for monitoring the density of electroscopic toner powder images wherein an electrically conductive light transmissive probe is utilized as a xerographic photoreceptor simulator for cyclically developing and cleaning a toner powder image on the probe surface. The density of the electroscopic toner image on the probe surface is sensed at a predetermined time by passing a light beam from a lamp through the probe and developer material into a photosensor. The photosensor and light source are supported in a protective sealed housing to eliminate contamination of these components by the ambient conditions of the xerographic developing apparatus. The photosensor is coupled to a threshold or level detector to generate a control signal when the density of the image developed on the probe surface is less than a predetermined amount.

Kamola [451 Aug. 8, 1972 AUTOMATIC DEVELOPER CONTROLLER [72] Inventor:Roman C. Kamola, North Rose,

, NY. [73] Assignee: Xerox Corporation, Rochester, NY. [22] Filed: March3, 1969 [21 Appl. No.: 803,773

Primary Examiner-Peter Feldman Attorney-Paul M. Enlow, Donald F. Daley,James J. Ralabate, Norman E. Schrader, Ronald Zibelli and August E.Roehrig, Jr.

[57] ABSTRACT Apparatus for monitoring the density of eiectroscopictoner powder images wherein an electrically conductive lighttransmissive probe is utilized as a xerographic photoreceptor simulatorfor cyclically developing and cleaning a toner powder image on the probesurface. The density of the electroscopic toner image on the probesurface is sensed at a predetermined time by passing a light beam from alamp through the probe and developer material into a photosensor. Thephotosensor and light source are supported in a protective sealedhousing to eliminate contamination of these components by the ambientconditions of the xerographic developing apparatus. The photosensor iscoupled to a threshold or level detector to generate a control signalwhen the density of the image developed on the probe surface is lessthan a predetermined amount.

9 Clains, 6 Drawing figures PATENTEDMIB elm 3,682,132

' SHEET 1 OF 5 INVENTOR. ROMAN OKAMOLA W ATTORNE v PmENTE-Dws wn3,682,132

SHEEI 3 OF 5 PATENTED 3 I972 SHEET t 0F 5 FIG 6 PATENTED 81972 I3.682.132

sum 5 or 5 I I I I I I I I I I //5v ac AUTOMATIC DEVELOPER CONTROLLERBACKGROUND OF THE INVENTION This invention relates to the monitoring andcontrol of latent electrostatic image development and in particular to anovel xerographic image development sensor adapted to monitor thedensity of electroscopic toner powder images in a xerographic developingsystem for controlling the amount of toner powder in a xerographicdeveloper mixture.

More specifically, the invention relates to a pho toreceptor simulatorutilizing an electrically conductive light transmissive probe formonitoring the toner powder in a dynamic xerographic development system.The probe is utilized for cyclically developing and cleaning a tonerpowder image on the probe surface to sense the density of anelectroscopic toner powder image formed thereon by means of a light beampassed through the image bearing probe into a photosensor. The lightsource and photosensor are supported in a housing to eliminate spuriousdeposition of toner powder on these elements.

In the process of xerography, a xerographic plate comprising a layer ofphotoconductive material on a conductive backing is given a uniformelectrical charge over its surface and then exposed to the subjectmatter to be reproduced by various projection techniques. This exposuredischarges the plate in accordance with the light intensity reaching it,thereby creating a latent electrostatic image on or in the plate.

Development of the image is effected by developers which comprise, ingeneral, a mixture of suitable pigmented or dyed resin-based powder,hereinafter referred to as toner, and a granular carrier material whichfunctions to generate triboelectric charges on, and to carry the toner.More specifically, the function of the carrier material is to providemechanical control of the toner,'or to carry the toner to an imagesurface and simultaneously provide almost complete homogeneity of chargepolarity. In the development of the image, the toner powder is broughtinto surface contact with the photoconductive coating and is heldthereon electrostatically in a pattern corresponding to the latentelectrostatic image. Thereafter, the developed xerographic image may betransferred to a support material to which it may be fixed by anysuitable means such as heat fusing.

In the mixture of toner particles and carrier material, the tonerparticles, which are many times smaller than the carrier material,adhere to and coat the surface of carrier material due to thetriboelectric attrac' therebetween. During development, as thetonercoated carrier material rolls or tumbles over the xerographic platecarrying a latent electrostatic image of a polarity opposite to thecharge on'the toner, toner particles are pulled away from the carrier bythe latent electrostatic image and deposited on the plate to form adeveloped toner-powder image. As toner-powder images are formed,additional toner powder must be supplied to the developer mixture toreplenish the toner deposited on the xerographic plate. The tonermaterial may be of the type disclosed in Carlson US. Pat. No. 2,940,934,wherein the toner particles comprise a finely divided pigmented resinhaving a particle size less than 20 microns and preferably an averageparticle size between about 5 and microns and comprising a finelydivided uniform mixture of pigment in a non-tacky, low-melting resin.Desirably, the pigment will be a black pigment such as carbon black orother minutely divided carbonaceous pigment.

As the toner powder in the developer mixture is depleted during thedevelopment of the latent image on the xerographic plate, more tonerpowder must be added to maintain a desirable level of copy density. Inthe event that too much toner powder is added to the developer mixture,heavy deposits of toner in the image areas, in combination with anundesirable deposit of toner in the non-image or background areas,results in producing prints of poor contrast with blotchy images or poorresolution.

In addition, overtoning by the operator adds to the severity of tonerpowder accumulation on critical machine components such as thecorotrons, illumination system, optical system, fuser and transportsystem, as well as necessitating more frequent replacement of filterbags and cleaning brushes. Thus, with an automatic development controlsystem incorporated in an automatic xerographic reproducing machine tosense the density of toner powder images and thereby regulate the amountof the toner powder in the developer mixture, fewer service calls arenecessary to'keep the quality of the xerographic reproductions ata highlevel.

In automatic reproducing machines such as shown in FIG. 1 a movingxerographic plate, which may be in the form of a cylinder, is exposed toa light source to create a latent electrostatic image to be developed byappropriate means such as a continuous flow of developer material overthe plate surface. It is necessary, in order to produce prints ofconsistently good copy density, to vary thetoner dispensing rate inaccordance with the rate of consumption which is a function of the typeand frequency of copy being reproduced. The dispensing of toner in priorart devices has been dependent upon the machine operator visuallyinspecting the finished copy and manually regulating the tonerconcentration by appropriate changes in a machine setting. It is readilyapparent that dispensing by this means results in image densitieslargely dependent on the alertness and ability of the operator tovisually evaluate the density of the copy image. Not only must theoperator detect the need for a setting change, but the operator must beable to accurately effect the proper ,degree of change through thedispenser setting. In the event that the operator oversets thedispensing rate and excess toner is added to the developer mixture, theonly method whereby the toner concentration may again become acceptableis by the reproduction of a sufficient number of additional copies.Considerable waste of material and time usually occurs when the propersetting of toner concentration has to be determined by an operator,since setting changes are made only after copy deterioration has becomeapparent.

Various attempts have been made to develop a sensor which would controlthe development of a toner powder image on the photoreceptive surface ofthe xerographic plate or drum. It has been found that control systemswhich expose and develop a toner powder test image on a portion of thexerographic drum, which is sensed by reflecting a light beam from thetest image into a photosensor, cause fatigue of the photoreceptivematerial and eventually give erroneous determinations.

Another attempt to solve this problem was the use of a sensor tosimulate the photoreceptive surface of the xerographic drum. Such anapparatus is disclosed in U.S. Pat. No. 3,094,049 issued June 18, 1963,to Christopher Snelling. The apparatus disclosed in the aforementionedpatent utilizes a xerographic drum simulator having electricallyisolated portions with a series of graduated reference indiciacalibrated to correspond with toner deposition patterns of knownconcentration when the simulator (electrode) is biased with apredetermined voltage. A light beam is passed through the calibratedsimulator into a photosensor to control the amount of toner powder inthe developer mixture in response to the toner concentration determinedby the simulator. This apparatus of the aforementioned patent senses theconcentration of the toner powder in the developer mixture bycorrelating the amount of light passing to the photosensor through theportions of the simulator which do not have a toner powder image formedthereon. As more or less toner powder is attracted to the simulator, thelight transmitting portion of the electrode changes, thereby, varyingthe output of the photosensor.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto improve the development of latent electrostatic charge patterns.

Another object of this invention is to improve xerographic developing byautomatically adjusting the amount of toner powder in the developermixture in response to the density of an electroscopic toner powderimage.

A further object of this invention is to improve xerographic developingby cyclically developing and cleaning an electroscopic toner powderimage on a xerographic drum simulator and sensing the density of theimage developed thereon.

Still another object of this invention is to prevent erroneousdetermination of image density by protecting the sensor components fromspurious deposition of toner powder due to ambient conditions of thedeveloper apparatus.

These and other objects are attained in accordance with the presentinvention wherein there is provided a xerographic drum simulatorincluding a transparent electrically conductive probe having asubstantially uniform electrical charge on its surface and positioned incooperative relationship with the developer mixture of a xerographicreproducing machine. The probe is electrically charged to cyclicallydevelop and clean an electroscopic toner powder image by means of thequantity of developer material moving relative thereto. The sensorapparatus disclosed herein is adapted to generate a control signal inresponse to image density and is effectively sealed in a housingstructure to prevent spurious toner powder accumulation on the sensorcomponents.

DESCRIPTION OF THE DRAWINGS Further objects of this invention togetherwith additional features contributing thereto and advantages accruingtherefrom, will be apparent from the following description of oneembodiment of the invention when read in conjunction with theaccompanying drawings wherein:

FIG. 1 is a front elevational view of an automatic xerographicreproducing machine utilizing the invention of this application;

FIG. 2 is an enlarged front elevation of the developer apparatus of theautomatic xerographic reproducing machine with parts broken away tobetter illustrate the sensor apparatus of this invention;

FIG. 3 is a front perspective view of the sensor to better illustratethe components thereof;

FIG. 4 is a partial sectional view of the sensor taken along lines 4-4of FIG. 3;

FIG. 5 is a partial sectional view of the sensor taken along lines'S-Sof FIG. 3; and

FIG. 6 is a mechanical schematic of the sensor apparatus.

FIG. 7 is an electrical schematic of the sensor apparatus.

Referring now to the drawings, there is shown in FIG. 1 an embodiment ofthe subject invention in a suitable environment such as an automaticxerographic reproducing machine, although it should be noted that theinvention is not intended to be limited thereto.

The automatic xerographic reproducing machine includes a xerographicplate 14 including a photoconductive layer or light receiving surface ona conductive backing, journaled in a frame to rotate in the directionindicated by the arrow to cause the plate surface to sequentially pass aseries of xerographic processing stations.

For the purpose of the present disclosure, the several xerographicprocessing stations in the path of movement of the plate surface may bedescribed functionally, as follows:

A charging station 15 at which a uniform electrostatic charge isdeposited on or in the photoconductive plate;

an exposure station 13 at which a light or radiation pattern of copy tobe reproduced is projected onto the plate surface to dissipate thecharge in the exposed areas thereof to thereby form a latentelectrostatic image of the copy to be reproduced;

a developing station A at which the xerographic developing material,including toner particles having an electrostatic charge opposite tothat of the latent electrostatic image, is cascaded over the platesurface whereby the toner particles adhere to the latent electrostaticimage to form a toner powder image in configuration of the copy beingreproduced;

a transfer station B at which the toner powder image iselectrostatically transferred from the plate surface to a transfermaterial or a support surface; and

a drum cleaning and discharge station C at which the plate surface isbrushed to remove residual toner particles remaining thereon after imagetransfer, and exposed to a relatively bright light source to effect sub-5. stantially complete discharge of any residual electrostatic chargeremaining thereon or therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is felt that the precedingdescription of the xerographic process is sufficient for a betterunderstanding of this invention. Referring now to the subject matter ofthe invention as best shown in FIGS. 2-5, the developing station A whicheffects development of the latent electrostatic image of the cylindricalxerographic plate, comprises a developer apparatus 16 which coacts withthe cylindrical plate 14 to develop the latent electrostatic'image on orin the plate surface by means of the xerographic toner powder. Mountedwithin the developer housing is a driven bucket-type conveyor 18 used toconvey developer material from the reservoir portion of the developerhousing to the upper portion of the developer housing from where it iscascaded over a hopper chute onto the xerographic drum. As shown, theconveyor includes a series of parallel spaced buckets 19 secured to asuitable pair of conveyor belts passing around a conveyor drive pulleyand a conveyor idler pulley secured on appropriate drive and idlershafts to rotate therewith. As the developer material cascades over thedrum, toner particles of the developer material adhere electrostaticallyto the previously formed latent electrostatic image areas on the drum14, the remaining developer material falling off the peripheral surfaceof the drum to be deflected by a baffle plate into the bottom of thedeveloper housing. Toner particles consumed during the developingoperation to form the visible powder images are replenished by a tonerdispenser 17 mounted within the developer housing. For further detailsconcerning the specific details of construction for a suitable developerapparatus reference is made to W. G. Lewis et a]. U.S. Pat. No.3,067,720.

As the developing mixture is cascaded over the xerographic drum, tonerparticles are pulled away from the carrier and deposited on the drum toform toner powder images, while the partially denuded carrier particlespass off the drum into the reservoir. As toner powder images are formed,additional toner particles must be supplied to the developing mixture inproportion to the amount of toner deposited on the drum. To supplyadditional toner particles to the developing mixture, the tonerdispenser 17 is used to accurately meter toner to the developer mixture.Although any one of a number of well-known powder or granulated materialdispensers may be used, the toner dispenser shown (FIGS. 1 and 2) is ofthe type disclosed in R. A. Hunt U.S. Pat. No. 3,013,703.

The toner dispenser 17 dispenses a uniform quantity of toner for a givenstroke of the dispensing plate and, therefore, it is apparent that thequantity of toner delivered by the toner dispenser may be altered byvarying the number of strokes per unit of time. Reciprocation of thedispensing plate is effected by means of an eccentric secured to the endof a shaft coacting with a bifurcated lever arm secured to thedispensing plate. It is felt that the preceding description of the tonerdispenser 17 is sufficient for an un derstanding of its function inrelation to the subject matter of this invention. For further detailsrelative to the specific construction of this device, reference is madeto the aforementioned Hunt Patent.

In order to control the dispensing of toner from the toner dispenser 17,there is shown in FIGS. 2-7 the details of an electroscopic toner powderimage density sensor 40 which ultimately efie'ctuates actuation of thedispenser by energization of the dispensing motor M in accordance withthe density of an image developed on the surface of a probe 44.

The xerographic image density sensor 40, to be hereinafter described indetail, is secured within the developer housing 16 by a suitable bracket26 appropriately fixed to a side wall of the developer housing whichelectrically insulates the sensor from the surrounding structure. Acollecting hopper 41 having side portions adapted to contain a quantityof xerographic developer material is secured to a support member 27 ofthe sensor for maintaining the collecting hopper beneath the movingbuckets 19 of the conveyor system to receive the overflow of developermaterial falling from each bucket as it progresses to cascade thedeveloper material onto the xerographic plate. Although the overflow ofdeveloper material is usually of a sufficient quantity, it is sometimesdesirable to provide small holes 29 in the bottom of the buckets 19 toincrease the amount of developer material passed to the sensor. Thecollecting hopper 41, as shown in FIG. 2, is positioned at an angle ofapproximately 20 relative to the vertical in such a way as to catch thefalling developer material and to guide the material into the sensorapparatus 40. The front wall 25 of the hopper 41 is slidably secured tothe side portions of the hopper to permit adjustment of the throat 24formed between the front wall and the support member 27 to set the flowrate of the developer material through the sensor.

- The sensor apparatus 40 (FIGS. 2-5) includes a photoelectric sensorP-l positioned in the path of light emanating from lamp L-l to vary theresistance of the photosensor P-l in proportion to the light intensityimpinging thereon. The photosensor P-l is electrically connected in aconventional circuit whereby the change in resistance of the photosensoris interpreted as a change in voltage. The light source L1 is supportedwithin the upper portion 40 a of sensor housing 40 by a set screw in aposition to direct a light beam into the photosensor P-l. Interposed inthis light path emanating from lamp L-l to the photosensor P-l, are apair of electrically conductive transparent electrodes 42 and 43. Asuitable type of electrode is a tin oxide coated glass platemanufactured by Pittsburgh Plate Glass Inc. under the trade name of NESAGlass. As shown, the lamp L-l and photosensor P-l are positioned suchthat the light beam is passed directly from the light source through theprobe 44 into the photosensor. However, it is obvious that a mirrorsurface would be utilized, if desired, to form a folded optical pathsuch that the light beam would be reflected through the image on theprobe surface and into the photosensor. The transparent lighttransmitting surface of the probe 44 is intended to include either thetransparent property of the probe or its reflective property asheretofore discussed.

Electrode 43 has a portion through which the light beam from lamp L-lpasses electrically charged to a polarity the same as that of the chargeon the toner powder to repel the toner powder from the electrode surfacefor keeping the transparent surface clear of ambient toner powder.Alternatively, the entire surface of electrode 43 may be charged torepel the toner powder. This electrode is secured in the sensor housing40 a by means of any suitable commercially available sealant, such asG.E. RTV adhesive sealant, to preclude the normal toner powder dustatmosphere within the developer housing 16 from depositing on the lampL-l. The sealing of the lamp L-l in the upper housing 40 a preventserroneous measurements of the image density as a result of spuriousdeposition of toner powder on the light source.

As best shown in FIGS. 3 and 5, the upper sensor housing 40 a isslidably supported on bar 21 which is fixedly secured to the lowersensor housing 40 b at its opposite end to permit adjustment of thespacing between the electrodes 42 and 43 to allow the quantity ofdeveloper material passing through the throat 24 and across probe 44from being constricted by the electrode surfaces.

The electrode 42 is supported in the lower housing portion 40 b ofsensor 40 and sealed in this position by a suitable sealant, such asthat previously described, to preclude the normal toner powder dustatmosphere within the developer housing 16 from depositing on thephotosensor P-l. The sealing of the photosensor in the housing preventsinterference with the measurement of the density of the toner powderimage as a result of spurious deposition of toner powder on thephotosensor. Electrode 42 is formed with a light transmitting surface orprobe 44 carrying a toner attracting charge of one polarity which isuniformly distributed over the probe surface. The light .transmittingprobe 44 is electrically connected by means of suitable contacts 20which electrically couple the probe to first and second switch contacts47 and 48, respectively. A portion 45 of the electrode 42 adjacent tothe probe 44 is connected by means of contacts 20 which electricallycouple portion 45 to terminals 49 and 50. The contacts are part of adouble pole single throw switch having a first switch arm 51 movablebetween the contacts 48 and 49 and a second switch arm 52 movablebetween the contacts 47 and 50.

To provide a more accurate determination of the image density developedon the surface of probe 44, and to minimize the response time requiredto add the desired amount of toner powder to the developer mixture, theelectrical charge on the probe surface is cyclically varied to attracttoner powder to and repel toner powder from the probe surface. When thepolarity of the probe 44 is reversed to repel the toner powder, thecontinuous cascade of developer material over the probe surface erasesthe previously formed image. In operation, each of the switch arms 51and 52 are connected to a pole of a source 53 of direct current forenergizing portion 45 of the electrode 42, probe 44, the light sourceL-l and to continuously maintain the toner repelling charge on electrode43. These connections are shown in FIG. 6 with the positive pole ofsource 53 being connected to the arm 52 thereby providing the probe 44with a toner attracting charge which is uniformly distributed over itsentire surface. The negative pole is connected to the switch arm 51thereby providing the electrically isolated portion 45 with a negativepotential. This electrical configuration is merely illustrative and hasbeen chosen for descriptive purposes because of the particular chargechosen for the toner particles, which for purposes of illustration, isnegative. The electrical coupling of the switch arms 51 and 52 is suchthat toner particles will be attracted to the probe 44 and repelled fromthe upper electrode 43 and the portion 45 of the electrode 42. Theelectrical charge on the probe 44 and the surrounding portion 45 of theelectrode 42 is cyclically alternated to attract and repel toner powderfrom the probe surface. During the portion of the cycle in which theprobe 44 is charged positively, toner powder is attracted from thedeveloper mixture to create a toner powder image over the probe surfaceand the density of the image so formed is sensed by means of the changeof resistance of the photocell due to the variation in the amount oflight impinging thereon. When the polarity of the electrical charge onthe probe 44 is reversed, the toner powder will be repelled from theprobe surface and the probe thereby cleaned by means of the continuouscascade of developer material across the electrode surface. Thephotosensor is deactuated during this portion of the cycle.

To provide this cyclic operation, the switch arms 51 and 52 aremechanically connected together and to the armature of a solenoid 54. Aswitchable timing device 55 is connected to the solenoid forperiodically energizing the solenoid to effect movement of its plunger.With the switch arms 51 and 52 positioned in contact with terminals 49and 47, respectively, toner particles are attracted from the developermixture to the probe 44. After a predetermined period of time haselapsed, preferably 10 seconds, the timer will energize the solenoid 54for switching the arms 51 and 52 to the terminals 48 and 50,respectively, and to open a switch, not shown, to inactivate thephotosensor P-l to preclude sensing during the cleaning cycle. Thisreversing of the charge on the probe 44 repels the accumulated tonerpowder and the developer material passing over the probe surface duringthis cycle cleans the probe to thereby condition the probe surface foranother attract cycle. As previously stated, the upper electrode 43 iscontinuously charged with a negative polarity to repel toner powder fromthe electrode surface to insure an accurate determination of the densityof the image formed on the probe 44.

Within the sensor 40, photosensor P-l is positioned directly below probe44 and supported in the light path passing directly from lamp L-lthrough the probe 44. Due to the conventional electrical connection, theoutput from the photosensor, therefore, is controlled by the density ofthe toner powder image developed on the surface of probe 44. The outputfrom the photosensor is coupled to a suitable threshold detector, suchas a conventional Schmidt trigger 66, connected to an amplifier 67 whichis utilized to amplify the signal produced by the threshold detector andto energize a solenoid 68 for actuating a normally open switch S-lconnected in series with a normally closed switch 8-2. When the outputfrom the photosensor P-l, coupled to the threshold detector 66, is of apredetermined magnitude, the detector will provide an electrical signalwhich will result in additional toner powder being added to thedeveloper mixture.

and are connected to a suitable source of electrical power. Uponenergization of the solenoid 68, the switch S-l closesto causeenergization of the motor M. The motor shaft, or a mechanical device 32,may be provided with a cam arrangement (not shown) adapted to actuatethe normally closed limit switch -2 to an open position during a portionof each revolution of the motor shaft. With solenoid 68 beingcontinuously energized during an under-toned condition of the developermixture, the motor will intermittently actuate the mechanical device 32.

During normal operation of the automatic toner dispensing apparatus, thesensor light source L-l is continuously energized for passing a lightbeam through the probe 44. This light is transmitted through the imageon the probe 44 and sensed by the photocell P-l. The firing level of thethreshold detector 66 is adjusted so that there is an output from thedetector as determined by the desired density of the image that isformed on the probe 44. As the toner supply in the developer housing 16depletes during normal xerographic processing, the density of the tonerimage that is formed on the probe 44 will lessen. With an image densitylower than the predetermined level the Schmidt trigger, which in effectis a level or threshold detector, will produce an electrical pulsecausing a momentary energization of the solenoid and closing the contactS- 1. For each electrical pulse produced in this manner, the motor shaftfor motor M will rotate one complete revolution to actuate thedispensing of toner powder into the developer mixture.

As the density of the toner image on the probe 44 increases, the outputfrom the level detector 66 will become restored thereby terminatingfurther actuation of the toner dispenser. The sensitivity of the sensingcircuit can be varied by adjusting the firing level of the thresholddetector 66 and the strength of the D. C. source 65. These componentswill determine the output to the solenoid and may be varied so that ahigh unbalance must be present before a level can be detected by theSchmidt trigger thereby obtaining a relatively wide density range forxerographic reproductions. If high quality contrast is needed in thereproductions, then a very'sensitive level may be preset whereby theslightest unbalance will demand toner dispensing and replenishment.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teaching of the inventionwithout departing from its essential teachings.

What is claimed is:

1. An electroscopic toner powder image sensor to monitor the amount oftoner powder in an electroscopic developer mixture including a lighttransmissive probe having a toner attracting charge which is uniformlydistributed over the surface, said charge being capable of attracting auniformly deposited toner powder image on said surface from a mixture ofelectroscopic developer material in a quantity indicative of the amountof toner powder in the developer mixture,

means for moving the electroscopic developer mixture and probe relativeto one another to attract and retain the toner powder on said probesurface,

sensing means for detecting the amount of toner retained on said surfaceto monitor the amount of toner powder in the developer mixture forproducing a control signal, and

a housing enclosing said sensing means, said housing including a lighttransmissive conductive member electrically biased to preventelectroscopic toner powder from being deposited on a surface thereofthrough which light is transmitted.

2. An electroscopic toner powder image sensor including means defining aflow path for passing electroscopic developer material comprising tonerparticles and carrier material therethrough,

a light transmissive probe positioned adjacent said flow path and incontact with passing developer material,

means operatively connected to said probe for forming toner particleimage on the probe surface,

sensing means responsive to the toner particle image formed on the probesurface for producing an electrical control signal, and

housing means enclosing said sensing means, said housing means includinga light transmissive conductive member biased to prevent electroscopicdeveloper material from being deposited on a surface thereof throughwhich light is transmitted.

3. The apparatus of claim 2 wherein said sensing means is responsive tothe density of the toner particle image formed on the probe surface.

4. The apparatus of claim 3 wherein said control signal is coupled tomeans for adding toner particles into the developer material.

5. The apparatus of claim 3 wherein said means operatively connected tosaid probe for forming a toner particle image on the probe surface iscyclically operable to form and erase a toner particle image from theprobe surface.

6. The apparatus of claim 3 wherein an electrode is positioned tointercept the passing developer material to effect a cascading of thedeveloper material over the electrode surface.

7. The apparatus of claim 3 including means to retain an amount ofdeveloper material for passing over said light transmissive probe at apredetermined flow rate.

8. The apparatus of claim 3 wherein said sensing means includesilluminating means positioned to pass a light beam through said probe,and passing developer material, and

a photosensor positioned in the light beam of said illuminating meanswith the probe and passing developer material therebetween.

9. An electroscopic toner powder image sensor including means defining aflow path for passing electroscopic developer material comprising tonerparticles and carrier material therethrough,

a first member positioned adjacent said flow path and in contact withpassing developer material,

housing means enclosing said sensing means including a lighttransmissive conductive member electrically biased to preventelectroscopic developer material from being deposited on a surfacethereof through which light is transmitted.

1. An electroscopic toner powder image sensor to monitor the amount oftoner powder in an electroscopic developer mixture including a lighttransmissive probe having a toner attracting charge which is uniformlydistributed over the surface, said charge being capable of attracting auniformly deposited toner powder image on said surface from a mixture ofelectroscopic developer material in a quantity indicative of the amountof toner powder in the developer mixture, means for moving theelectroscopic developer mixture and probe relative to one another toattract and retain the toner powder on said probe surface, sensing meansfor detecting the amount of toner retained on said surface to monitorthe amount of toner powder in the developer mixture for producing acontrol signal, and a housing enclosing said sensing means, said housingincluding a light transmissive conductive member electrically biased toprevent electroscopic toner powdEr from being deposited on a surfacethereof through which light is transmitted.
 2. An electroscopic tonerpowder image sensor including means defining a flow path for passingelectroscopic developer material comprising toner particles and carriermaterial therethrough, a light transmissive probe positioned adjacentsaid flow path and in contact with passing developer material, meansoperatively connected to said probe for forming toner particle image onthe probe surface, sensing means responsive to the toner particle imageformed on the probe surface for producing an electrical control signal,and housing means enclosing said sensing means, said housing meansincluding a light transmissive conductive member biased to preventelectroscopic developer material from being deposited on a surfacethereof through which light is transmitted.
 3. The apparatus of claim 2wherein said sensing means is responsive to the density of the tonerparticle image formed on the probe surface.
 4. The apparatus of claim 3wherein said control signal is coupled to means for adding tonerparticles into the developer material.
 5. The apparatus of claim 3wherein said means operatively connected to said probe for forming atoner particle image on the probe surface is cyclically operable to formand erase a toner particle image from the probe surface.
 6. Theapparatus of claim 3 wherein an electrode is positioned to intercept thepassing developer material to effect a cascading of the developermaterial over the electrode surface.
 7. The apparatus of claim 3including means to retain an amount of developer material for passingover said light transmissive probe at a predetermined flow rate.
 8. Theapparatus of claim 3 wherein said sensing means includes illuminatingmeans positioned to pass a light beam through said probe, and passingdeveloper material, and a photosensor positioned in the light beam ofsaid illuminating means with the probe and passing developer materialtherebetween.
 9. An electroscopic toner powder image sensor includingmeans defining a flow path for passing electroscopic developer materialcomprising toner particles and carrier material therethrough, a firstmember positioned adjacent said flow path and in contact with passingdeveloper material, means operatively connected to said member forforming a toner particle image on a surface thereof, sensing meansresponsive to the toner particle image formed on the surface forproducing an electrical control signal, and housing means enclosing saidsensing means including a light transmissive conductive memberelectrically biased to prevent electroscopic developer material frombeing deposited on a surface thereof through which light is transmitted.